Rotary impact tools in which output portions are rotated by blow and impact from hammers that are driven so as to rotate by motors, or rotated by pulse impact produced by oil pressure, are widely used in construction sites and assembly plants since higher torque can be obtained by the impact being applied, as compared to rotary tools that simply use decelerators.
However, since the rotary impact tool has a high torque characteristic that a high torque can be obtained by one impact, a subject to be tightened tends to be excessively tightened, and the subject may be thus damaged. If an operator relatively loosely tightens a subject to be tightened lest the subject should be excessively tightened, a problem may arise that, for example, tightening torque becomes insufficient, and the subject cannot be fixed as intended.
In order to appropriately tighten a subject, for example, Japanese Laid-Open Patent Publication No. 8-267368 discloses that a torque sensor is mounted to an output shaft, and when a torque measured by the torque sensor reaches a target torque, a motor is halted.
On the other hand, in the rotary impact tool, an increase of torque for one impact is set so as to be great in order to quickly tighten a large bolt. However, if a small bolt is tightened by using the same tool, a torque may become higher than or equal to a target torque having been set, by only one impact being applied, or may become higher than torques within a range of the target torque even when the number of times an impact is applied is quite small. Therefore, a problem may arise that an accuracy of the tightening torque is reduced since it is difficult to halt the motor after an appropriate torque has been generated. Thus, a plurality of tools need to be selectively used depending on a magnitude of the target torque.